Enamel is the hardest biologic tissue found in the body and it forms a protective layer covering the crowns of teeth. It is composed of interlocking rods which, to more than 96%, are constituted by hydroxyapatite. The rods are deposited in keyhole shapes each of which comprise a head and a tail surrounded by a sheath (cf. FIG. 1 as enclosed). Enamelin, a protein unique to enamel, can be found in minute amounts surrounding individual hydroxyapatite crystallites predominantly in the sheath (for a review see Avery J K. Essentials of Oral Histology and Embryology. A Clinical Approach. St. Louis, Mosby, 1992).
The body of the tooth, both the crown and the root thereof, is constituted by dentin. Dentin is composed of an organic matrix of collagen fibers (on average 20% by weight), in which hydroxyapatite crystallites (on average 70% by weight) are dispersed. The remaining 10% of the dentin is constituted by water. Dentin tubules, approximately 1 .mu.m in diameter and of a density amounting to 30 000 to 50 000 tubules per mm2, run from the centrally located pulp to the periphery of the body of dentin (cf. FIG. 2 as enclosed). The walls of the tubules are made up of peritubular dentin which is approximately 40% more highly mineralized than the intertubular dentin in between the tubules. The water component of dentin is mainly found in the dentin tubules (for a view, see Avery 1992, above).
The bonding of dental restorations to dental mineralized tissues is the last step in filling therapy (for details, see Heymann, H., Bayne, S. Current Concepts in dentin bonding. Journal of the American Dental Association 1993, 124, 27-35). The process starts with mechanical removal of carious enamel and dentin followed by cavity preparation. The cavity walls are subsequently pretreated before insertion of the filling material in order to increase adherence of the material to the walls and to minimize gap formation. This process is generally referred to as bonding and relies on two principles:
Mechanical interlocking of the resin-based restoration to irregularities in the mineralized surface. PA1 Chemical bonding of the resin-based restoration to exposed collagen. PA1 remove the debris, such as smear and bacteria, resulting from the mechanical cavity preparation; PA1 maximize the surface area of the enamel cavity walls by eroding the more heavily mineralized enamel rod heads. This produces protruding ridges of rod sheaths for mechanical interlocking with resin-based restorations; PA1 expose collagen in the dentin surface to make the fibers accessible for chemical bonding to resin-based restorations.
The bonding usually involves an initial step of surface etching using ortho-phosphoric acid, such etching having for its purpose to:
Etching is, by definition, the selective removal of parts or components from a solid surface through the action of an etching agent, such as solutions of acids or other substances. Etching does not, however, implicate erosion of the surface to remove a complete surface layer. The purpose of the etching of exposed dental mineralized tissues after cavity preparation is not the same for all tissues involved. Thus, in regard to a dentin surface the purpose is to selectively remove smear and hydroxyapatite leaving an exposed layer of collagen. With regard to an enamel surface the purpose is to increase the surface area available for bonding by removing hydroxyapatite from the more highly mineralized enamel rods.